This invention relates to removal of conformal coatings from printed circuit boards, and, in particular, to a composition and method for removing polyurethane and epoxy coatings from printed circuit boards to permit and facilitate repair thereof.
A conformal coating, as that term is used herein, is a coating applied to a printed circuit board to protect the components and circuitry from moisture and contaminants, and, to some degree, from mechanical shock, vibration, shorting, and tampering. Although a large number of coating compositions have been used as conformal coatings, polyurethane and epoxy coatings are the most common. When properly applied, the conformal coating effectively surrounds the entire printed circuit board and the components thereon.
Typical printed circuit boards are formed of woven glass fibers impregnated with an epoxy resin and having etched or printed metal cladding on one or both sides defining electrical circuits. Holes are drilled in the circuit board to receive components, which are soldered into place and into electrical contact with the conductive metal circuits on the board.
Conformal coatings are becoming increasingly popular in consumer and business electronics products. However, their principal use is in applications in which the printed circuit board is likely to be subjected to moisture, shock, and abuse.
In order to effectively fulfill their function, conformal coatings must be impervious to water and most common solvents and not susceptible to ready removal. These properties, although desirable from the standpoint of protection of the circuit board and components, create significant problems in effecting repair of circuit boards containing defective components. This problem is discussed at length in U.S. Pat. No. 4,451,523 to Native, et al. Epoxy resins have been virtually impossible to remove chemically because any solvent that would remove the epoxy coating would, as a general rule, also damage the epoxy-resin impregnated circuit board and the potted electronic components on the circuit board.
The thickness of conformal coatings may vary from application to application. A conformal coating remover that can satisfactorily remove a 10 mil conformal coating may damage the board when used to remove a 2 mil conformal coating. The prior art has provided no effective solution to this problem.
As a result, repair of boards with epoxy conformal coatings has been a time-consuming and labor-intensive process. The only satisfactory method, to date, for repairing such coated boards is to physically abrade or scrape the coating off of the soldered junctions to permit desoldering and removal of defective components. The scraping or abrading processes for mechanically removing the conformal coating carry with them the possibility of damaging the components or the circuits on the board. If a circuit is cut, the board must be discarded.
Polyurethane coatings are also insoluble in most common solvents. Corrosive strippers which can remove the cured polyurethane coating will, as a general rule, also corrode metallic surfaces on the board. Thus, polyurethane coatings, like epoxy coatings, are typically removed by mechanical processes such as scraping and abrading to permit rework of the board.
There have been great advances in recent years in automated in-circuit testing equipment for printed circuit boards. Such automated testing machines can analyze and test in the neighborhood of 250 boards per hour. However, because of the dielectric properties of conformal coatings, these machines cannot be used on coated boards without first removing the conformal coating. Ironically, this step is still performed by hand, one board at a time.
Accordingly, there is a significant need for compositions which can satisfactorily remove epoxy and polyurethane conformal coatings without damaging the printed circuit board or the components thereon. One attempt to provide a conformal coating solvent mixture is described in U.S. Pat. No. 4,383,867 to Elwell. This patent discloses a mixture of 70% dichloromethane, 20% dimethyl formamide, and 10% methanol that is said to be useful in removing polyurethane conformal coatings from printed circuit boards. This single-part composition merely softens the polyurethane coating, which must still be scraped off. Unfortunately, it also softens the underlying resin-impregnated board. In addition, this composition is not suitable for use on epoxy-coated boards.
Another single-part composition for removing conformal coatings is disclosed in U.S. Pat. No. 3,625,763, to Melillo. This composition is said to loosen the resin in a few minutes, with complete removal occurring over a two-hour period. Two hours, of course, is much too long for practical utility. A practical conformal coating remover will remove conformal coating in 10 minutes or less, preferably in five minutes or less.
Accordingly, it is an object of the present invention to provide a composition and method for effectively removing cured epoxy and polyurethane conformal coatings from printed circuit boards.